Java XML appendChild

Hello!

I use org.w3c.dom (and I cant use any other) and now I need to be able to update a node and insert a node in a existing document.
I get the Document from a file with nodes like below. And I would for eg. be able to update the number node with a value since it is empty. But I can't figure out how since the Node in the this package is a Interface.
<student>
        <firstname>John</firstname>
        <lastname>Doe</lastname>
        <number></number>
</student>

I have tried something like this below where I want to update node values from som text fields.:
            for(int i = 0; i < textLabels.length; i++)
            {
                for(int j = 0; j < nodeList.getLength(); j++)
                {
                    Node tempNode = nodeList.item(j);
                    if(tempNode.getNodeName().equals( textLabels[i].getText() ))
                    {

                        if(tempNode.getFirstChild() != null)
                        {
                            tempNode.getFirstChild().setNodeValue( valueTextFields[i].getText() );
                        }
                        else
                        {
                            //tempNode.setNodeValue( valueTextFields[i].getText() );
                        }
                        System.out.println(textLabels[i].getText() + " " + valueTextFields[i].getText() + " " + tempNode.getFirstChild().getNodeValue());
                    }
                }
            }

Best regards

Fredrik